... but when I check stream the user, https://www.twitch.tv/thespiffingbrit/videos ... hasn't streamed in over a year. He has no videos to watch. 

Yesterday, amplitude had a stream where you could unlock everything by watching them(they really should have announced a reminder for it), but... obviously it’s over. Right now, as of this post, there’s another streamer streaming with drops which you can go and unlock her, unless she finished streamer after i posted this.

Yep you cant get Spiff via his stream since he dont use Twitch you need to wait for the devs to do a new unlock all avatars events for him.